1. Field of the Invention
The present invention relates to a process for preparing isobutene by dissociation of methyl tert-butyl ether (MTBE).
2. Description of the Related Art
Isobutene is a starting material for the production of many products, e.g. for the production of butyl rubber, polyisobutylene, isobutene oligomers, branched C5-aldehydes, C5-carboxylic acids, C5-alcohols and C5-olefins. It is also used as an alkylating agent, in particular for the synthesis of tert-butylaromatics, and as intermediate for the production of peroxides. In addition, isobutene can be used as precursor for the preparation of methacrylic acid and its esters.
In industrial streams, isobutene frequently occurs together with saturated and unsaturated C4-hydrocarbons. Owing to the small boiling point difference or the very low separation factor between isobutene and 1-butene in a distillation, isobutene cannot be separated economically from these mixtures. Isobutene is therefore usually isolated from industrial hydrocarbon mixtures by converting isobutene into a derivative which can easily be separated off from the remaining hydrocarbon mixture and redissociating the isolated derivative to form isobutene and the derivatizing agent.
Isobutene is usually separated off from C4 fractions, for example the C4 fraction from a steamcracker, as follows. After removal of the major part of the multiply unsaturated hydrocarbons, mainly butadiene, by extraction/extractive distillation or selective hydrogenation to linear butenes, the remaining mixture (raffinate I or hydrogenated cracking C4) is reacted with alcohol or water. The use of methanol results in formation of methyl tert-butyl ether (MTBE) from isobutene and the use of water results in tert-butanol (TBA). After they have been separated off, both products can be dissociated to form isobutene in a reversal of their formation.
MTBE is cheaper than TBA because the reaction of isobutene-containing hydrocarbons with methanol is easier than with water and MTBE is produced in large quantities as a component of 4-stroke fuels. The production of isobutene from MTBE is therefore potentially more economical than from TBA if a similarly good process were to be available for the dissociation of MTBE as for the dissociation of TBA.
The dissociation of MTBE can be carried out in the liquid phase, the gas/liquid phase or the gas phase in the presence of acid catalysts. Regardless of the phase in which the dissociation is carried out, by-products are formed, although to differing extents. For example, undesirable C8 and C12 components can be formed by acid-catalysed dimerization or oligomerization of the isobutene formed in the dissociation. The undesirable C8 components are mainly 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene. In addition, part of the methanol formed in the dissociation can be converted into dimethyl ether.
For economical reasons (low price; good availability), the preparation of isobutene by dissociation of MTBE is not carried out using high-purity MTBE, for example MTBE/S from Oxeno Olefinchemie GmbH, but is carried out using MTBE of standard industrial quality (fuel grade). Industrial MTBE usually contains C8 olefins, for example those mentioned above, 2-methoxybutane formed from linear butenes and methanol in the synthesis of MTBE, TBA, methanol and possibly C4- and C5-hydrocarbons as secondary components.
Various processes for preparing isobutene by dissociation of MTBE are known.
In EP 0 302 336, isobutene is obtained by dissociation of MTBE in a column which is a combination of a stirred vessel, a distillation column and an extraction column. The acid catalyst is located in the bottoms circuit. MTBE is fed into the bottoms circuit. Part of the circulating bottoms is taken off. The crude isobutene is extracted with water in the column. The aqueous, methanol-containing extract is taken off above the bottom. The isobutene is obtained as overhead product. The bottoms taken off are recirculated after discharge of a proportion thereof.
EP 0 068 785 claims a process in which the dissociation of MTBE is carried out in a continuously operated stirred vessel. Here, the acid catalyst is suspended in the starting material. Isobutene is separated off from the reaction mixture which distils off by means of a column from which it is obtained as overhead product. Part of the bottom product is recirculated to the stirred vessel. The other part is separated in a two-column system into an MTBE-containing stream which is recirculated to the stirred vessel and methanol which is taken off as a side stream. The way in which secondary components in the MTBE used and by-products formed are separated off is not disclosed.
In DE 32 10 435, MTBE is dissociated in a reactive distillation column. An isobutene mixture containing small amounts of methanol and traces of diisobutene is obtained as overhead product. The bottom product from the reactive distillation column is separated in a distillation column into an MTBE-containing stream which is recirculated to the reactive distillation column and a bottom product comprising methanol. The way in which by-products are separated off is not indicated.
The documents EP 0 633 048, DE 100 20 943 and DE 101 11 549 claim the preparation of isobutene by dissociation of MTBE in a reactive distillation column. The separation of secondary components from the dissociation mixture is not disclosed.
DE 102 27 350 and DE 102 27 351 describe processes for preparing isobutene by dissociation of MTBE in the gas phase. In both processes, the dissociation product is condensed and extracted with water. A water/methanol mixture is obtained as extract and is separated into methanol and water by distillation. The raffinate comprises isobutene, unreacted MTBE and secondary components. This mixture is separated by distillation into an overhead product which contains isobutene together with small amounts of dimethyl ether and a bottom product comprising MTBE and secondary components. No information is given about the use or work-up of the MTBE mixture obtained.
U.S. Pat. No. 6,049,020 describes, inter alia, the preparation of isobutene by dissociation of MTBE. Methanol is removed from the reaction product by extraction with water. The remaining raffinate is separated by distillation into an overhead product containing isobutene and a bottom product comprising unreacted MTBE and secondary components. The work-up of the MTBE mixture is not described. In U.S. Pat. Nos. 6,072,095 and 6,143,936, the work-up of the dissociation product is carried out analogously. The MTBE mixture containing the secondary components which is obtained is not worked up; it can be fed to a plant for preparing MTBE.
In the dissociation of industrial MTBE which contains 2-methoxybutane, linear butenes are formed by dissociation of 2-methoxybutane. None of the abovementioned processes provides a solution to the separation of linear butenes from isobutene or a limitation of the content of linear butenes in the isobutene fraction.
In Catalysis Today 34 (1997), pages 447 to 455 (Case history: synthesis and decomposition of MTBE), R. Trotta and I. Miracca demonstrate the dependence of the content of linear butenes in isobutene which has been produced by dissociation of MTBE on the 2-methoxybutane content of the feed MTBE and on the MTBE conversion. The relationship is shown in a graph which does not have a scale. The higher the 2-methoxybutane content of the feed MTBE and the higher the MTBE conversion, the higher the content of linear n-butenes in the dissociation product. It is not possible to prepare isobutene having a content of linear butenes of less than 1000 ppm by mass from a feed MTBE having a content of 2-methoxybutane (MSBE) of over 1000 ppm by mass at quantitative MTBE conversion. In this case, the MTBE conversion has to be reduced. This has the consequence that a relatively large amount of a mixture which comprises mainly methanol and MTBE and has a higher concentration of 2-methoxybutane than the feed MTBE is obtained after the isobutene has been separated off. This mixture must not, because of its relatively high 2-methoxybutane content, be fed into the dissociation reactor if the isobutene specification is to be adhered to. Even recirculation of this stream to an upstream MTBE plant which produces just the amount of MTBE required for the dissociation is not possible, since 2-methoxybutane would accumulate in the plant system. A purge stream associated with MTBE losses is therefore unavoidable.